Thermal processing techniques are commonly used to convert feedstock hydrocarbon material to more valuable products. For example, various thermal processing techniques are used to convert methane directly to C2 hydrocarbons, such as acetylene via reaction (1), ethylene via reaction (2), and ethane via reaction (3).2CH4→C2H2+3H2  (1)2CH4→C2H4+2H2  (2)2CH4—C2H6+H2  (3)
These reactions are highly endothermic, requiring 377 kJ/mol, 202 kJ/mol, and 65 kJ/mol, respectively. In addition, higher temperatures are generally required to achieve high conversion of the feedstock and high selectivity to the desired product.
One type of thermal processing used in the prior art involves exposing the feedstock to high temperature combustion gases causing the feedstock to pyrolyze into the desired unsaturated product. Many traditional processes involve steam cracking. Other processes involve combustion to generate the necessary temperatures.
The formation of acetylene from methane by thermal processing is difficult because of the relative free energies of formation of methane and acetylene. Acetylene and ethylene can continue reacting to form higher dienes and alkynes such as monovinylacetylene, and aromatic and polyaromatic compounds which can form undesirable tar and soot. Above 800 K, CxHy compounds may undergo decomposition into carbon and hydrogen. Below 1500 K, the free energy of formation of methane is above that of acetylene. As such, the formation of methane, the final product of thermodynamic equilibrium, is favored over acetylene between the temperatures of 800 K and 1500 K. Above 1500 K, however, the free energy of formation of acetylene is lower than that of methane. As a result, the formation of acetylene is favored over that of methane. But, as the reactants are cooled below 1500 K, the thermodynamic equilibrium shifts back to methane, and the acetylene produced at the higher temperature will decompose and reform as methane. Acetylene and the other hydrocarbons can continue to react to form aromatic and polyaromatic species. When water and carbon dioxide are present acetylene can react to form carbon monoxide, which is a less valuable product than acetylene. The pyrolitic reaction of methane to form acetylene and other desired hydrocarbons has a high activation energy, while the decomposition reactions of acetylene have lower activation energy. Thus, the formation of acetylene is favored by reacting at high temperatures but with short controlled residence times that minimize consecutive reactions of acetylene with additional acetylene, hydrocarbons and oxygen containing species such as H2O, CO2 and O2.
Certain prior art processes involve combusting a fuel mixture to create a high temperature supersonic carrier stream. A fuel and oxidizer are combusted to produce a hot gas stream at a super-atmospheric pressure and supersonic velocity. Feedstock is injected into the supersonic hot gas stream to initiate the endothermic pyrolysis reactions.
These prior art processes, however, rely on the turbulence of the stream to mix the feedstock within the carrier stream. Increased uniformity of composition and increased uniformity of temperature within the stream during acetylene formation will result in increased conversion and selectivity for the desired product. In prior art processes, feedstock is injected uniformly via a single row of uniform injectors along the wall of the reactor and at a different temperature than the carrier stream. This creates a non-uniform distribution with a stream of highly concentrated, low temperature feedstock alongside the high temperature carrier stream. Prior art reactors therefore included a mixing zone of sufficient length to allow the turbulent flow to mix the feedstock with the carrier stream.
U.S. Publication 2014/0058179 describes a pyrolytic reactor comprising a fuel injection zone, a combustion zone adjacent to the fuel injections zone, an expansion zone adjacent to the combustion zone, a feedstock injection zone comprising a plurality of injection nozzles and disposed adjacent to the expansion zone, a mixing zone configured to mix a carrier stream and feed material and disposed adjacent to the feedstock injection zone, and a reaction zone adjacent to the mixing zone. The plurality of injection nozzles are radially distributed in a first assembly defining a first plane transverse to the feedstock injection zone and in a second assembly transverse to the feedstock injection zone. The mixing zone is needed ensure that combined carrier and feed streams are fully mixed. The presence of this mixing zone will increase the residence time in the reactor and will lead to less desirable products.
Accordingly, there remains a need for an improved pyrolytic reactor having higher conversion and selectivity for the desired product.